Decision Feedback Equalization (DFE) is based on the principle that once the value of the current transmitted symbol has been determined, the contribution of intersymbol interference (ISI) to future received symbols can be removed. DFE has a nonlinear feature that is due to a decision device that attempts to determine which symbol of a set of discrete levels was actually transmitted. Once the current symbol has been decided, a filter structure calculates the ISI effect it would tend to have on subsequent received symbols and, thereafter, compensate the input to the decision device for subsequent samples. This post-cursor ISI removal is accomplished by the use of, among other things, a feedback filter structure.
From one perspective, a purpose of an equalizer in general is to compensate for the backplane attenuation due to insufficient bandwidth. More particularly, the DFE works by actively cancelling out signal distortion based on the history of the received data. The DFE does this by removing signal energy that may have leaked from one bit to a following bit which cancels out post-cursor distortion caused by inter symbol interference. In a DFE, the magnitude of the tap coefficient requires either manual tuning or leverages a form of an autonomous adaptation scheme, for example. In certain modern applications that require ever increasing data rates, they also require high performance from their DFE adaptation circuitry. These modern applications, however, also have stringent silicon real estate demands where the DFE adaptation circuitry is preferred to be as small as possible.
In typical implementations, reduced steady state error is achieved by implementing DFE adaptation circuitry with high integration values. High integration values are achieved with traditional look-ahead or ripple counters that can have performance limitations with regard to operational frequencies or can require large silicon real-estate.